Letters of Rec.

I need some advice with letters of recommendation. So far, I've been able to get good letters from my clinical research professor, biology professor, and employer. I haven't taken many classes outside of the sciences, so therefore, have not gotten to know my non-science prof very well. I'm applying to med school this summer. Should I enroll in a non-science summer class so that i can get a letter of rec them that prof? If any of you know the specific requirements on letters for different school, that would be a lot of help too. Thanks!